docs: say how the pinned toolchain goes wrong quietly

CONTRIBUTING.md

@@ -11,6 +11,34 @@ cargo fmt --all --check
 
 CI runs exactly these, on Linux, macOS and Windows.
 
+## The toolchain, and one way it goes wrong quietly
+
+`rust-toolchain.toml` pins the compiler, and rustup honours it — but only if the
+`cargo` you run is rustup's shim. A `cargo` from somewhere else earlier on
+`PATH` does not read that file, so it builds with whatever version it happens to
+be.
+
+That produces a failure worth recognising, because it looks like a defect in the
+code and is not:
+
+```
+error[E0514]: found crate `serde` compiled by an incompatible version of rustc
+```
+
+It happens when `cargo` and `rustdoc` resolve to different releases — a
+distribution or a package manager can install one and not the other. Check with:
+
+```bash
+command -v cargo rustc rustdoc
+cargo --version && rustdoc --version
+```
+
+If they disagree, put rustup's shims first for the shell you build in:
+
+```bash
+export PATH="$HOME/.cargo/bin:$PATH"
+```
+
 ## What the review looks for
 
 - **A new guard is observed failing first.** A test that has never failed on the